1. Field of the Invention
This invention is in the field of separating non-ferrous metals such as lead and zinc from waste materials containing recoverable amounts of iron by controlled sintering conditions utilizing a layered arrangement in a sintering zone.
2. Description of the Prior Art
In blast furnace and sintering processes, as well as in steel production, iron-containing wastes occur in considerable amounts as a flue dust. The treatment of these materials for recovering the iron values can be carried out, for example, by means of incorporating the same into a sinter charge for a blast furnace. As a practical matter, such a procedure encounters difficulties because of the additional non-ferrous metals such as zinc and lead which are usually present. Zinc enters into a blast furnace process both from the ores employed and the additives as well as from the recycled zinc-containing blast furnace flue dust. Zinc carriers appear in steel production as an unavoidable contaminant in this iron scrap to be processed. Steel-containing coatings and anti-corrosion composites containing zinc also contribute additional sources of zinc.
The zinc component in the charge fed to a blast furnace, however, must be limited to minimize the occurrence of injurious zinc circulations which are caused by continuing cyclical vaporizations in the lower zones of the blast furnace, and condensation in the upper zones thereby causing the formation of zinc deposits. These deposits can cause damage in the area of the walls. In order to avoid these difficulties, the use of such iron and zinc-containing waste materials has been abandoned in some instances but they have to be stored at special dumping sites which provide substantial cost. The storage of these waste materials is not only uneconomical but also represents a distinct burden on the environment.
Pyrometallurgical methods for the elimination of zinc from circulation in a blast furnace sintering system are described in "Stahl-und Eisen", Volume 98, 1978, No. 20, pages 6 through 14. In this article, there is described a zinc removal which occurs during the sintering process. According to the disclosure, 90% of the zinc contained in a sinter mixture to which 30% coke has been added can be removed from the mixture using a reduced amount of air, i.e., under reducing conditions. There is a disadvantage, however, in that there is a decrease of the sinter yield. The creation of reducing conditions by means of incorporating a corresponding excess of coke distributed throughout the entire sintering material charge, however, also results in an increased proportion of iron oxide (FeO) and thus provides a decreased solidity of the sintered product produced. In addition, the formation of iron silicates subsequently results, and their reduction in the blast furnace requires an additional coke charge. Consequently, particularly in view of the economic feasibility of the overall process, this method for removing zinc components from iron-containing waste materials has not been considered satisfactory.